WWE Error: java.lang.NullPointerException

i have a local server, and i try to connect to it using WWW class, like this:

 public void POST (LoginData loginData)
 {
     WWWForm form = new WWWForm ();
     string serializedData = Serialize (loginData);
     form.AddField ("login_data", serializedData);    
     WWW www = new WWW (URL, form);
     StartCoroutine (MakeUserWaitForRequest (www));    
 }
     
 IEnumerator MakeUserWaitForRequest (WWW www)
 {
     yield return www;
 
     // check for errors
     if (www.error == null) {
     Debug.Log ("WWW Ok!: " + www.text);
     user = Deserialize<User> (www.text);
     } else {
         Debug.Log ("WWW Error: " + www.error);
     }    
 }   

in UnityPlayer it works fine, but on my device i get this debug log:

  I/Unity(12796): WWW Error: java.lang.NullPointerException
  I/Unity(12796): UnityEngine.Debug:Internal_Log(Int32, String, Object)
  I/Unity(12796): UnityEngine.Debug:Log(Object)
  I/Unity(12796): <MakeUserWaitForRequest>c__Iterator4:MoveNext() (at C:\PATH\UserManager.cs:42)
  I/Unity(12796):  
  I/Unity(12796): (Filename: C Line: 0)

line 42 in UserManager.cs is Debug.Log ("WWW Error: " + www.error); I have permissions for internet, i can access my server from browser. Any help?